Sprinkle the graham cracker crumbs over the center of 4 plates. Drizzle 1/3 of the melted dark chocolate over the crumbs.
Place a watermelon rectangle over the crumbs and chocolate on each plate. Drizzle 1/3 of the dark chocolate over the watermelon.
Sprinkle the marshmallows over the watermelon and drizzle the remaining chocolate over the marshmallows.